Recently, with the popularity of the VTR use of the television receiving set (hereinafter referred to as the TV) and the VTR jointly to do TV watching, and recording and reproducing by means of VTR is increasing.
On connecting the TV with the VTR, an antenna, TV and VTR are connected to one another, and signal switching thereamong is carried out by a signal switching device equipped within the VTR. The signal switching device, is generally employed in order to allow the use of either the TV or the VTR, for instance, the use of the TV only, i.e., the VTR not powered. General connection between the TV and the VTR in such a case as above is illustrated in FIG. 1.
In FIG. 1, 1 is an antenna, 2 is VTR, 3 is TV, 4 is a signal switching device equipped within VTR 2, 5 is a distributor, 6, 8, 10 are high-pass filters, 7, 9, 11 are switching diodes for switching between the signal from the antenna 1 and the signal from VTR 2 to be transmitted to TV 3, respectively; 12, 15 are tuners equipped within VTR 2 and TV 3 respectively, and 14 is a modulator within VTR 2. In the structure shown in FIG. 1, VTR 2 and TV 3 are put in use properly through manipulation of a control section (not shown) causing switching of the switching diodes 7, 9, 11.
For example, at the time of normal reception of television broadcasting, the switching diodes 7, 9, 11 are turned off, the signal from the antenna 1 is transmitted through the distributor 5 and the high-pass filters 6, 8, 10 and applied to the tuner 15 of TV 3. Or, at the time of reproducing a recorded picture, the switching diodes 7, 9 are turned on to attenuate the signal from the antenna 1 passed through the distributor 5 and, on the other hand, the switching diode 11 is also turned on to apply a video signal from a recording-reproducing circuit 13 through the modulator 14 to the tuner 15 of TV 3.
The important portion of an input circuit in the conventional tuners 12, 15 employed in FIG. 1 has such a structure as shown in FIG. 2.
In FIG. 2, 21 is an input terminal of the tuner, 22 is a branching filter for separating the television signal which is received at the input terminal 21 and includes mingled VHF and UHF waves, composed of a low-pass filter 23 and a high-pass filter 24. 25 is an input tuning circuit which composes a part of the VHF tuner and is composed of tuning coils 26, 27, a varactor diode 28, a switching diode 29 for switching the reception bands, and by-pass capacitors 30, 31, 32. 39 is an RF amplifier included in the VHF tuner section whose output 40 is connected to an input section of a double tuning circuit within the VHF tuner. 41 is a terminal for a tuning voltage to be impressed on the varactor diode 28 of the tuning circuit 25, this terminal 41 being connected also to varactor diodes employed in other tuning circuits, local oscillator circuits and the like (not shown) within the VHF tuner and UHF tuner 44. 42, 43, 45 are terminals for band-switching sources to switch the reception television channel over to low VHF channels, high VHF channels, or UHF channels, respectively. The VHF band switching operation is carried out by cooperation of the voltages to be impressed on the terminals 42, 43, the switching diode 29, and resistors 34, 35, 37. The resistor 36 and the diode 47 are not only relating to the band switching operation, but also serving as discharging means for a charged voltage of the by-pass capacitors 30. That is, on the band switching operation described hereinafter, at the time of switching from high VHF channel reception to low VHF channel reception, the switching diode 29 is changed from the forward biased state to the reverse biased state. When the switching diode 29 changes from the conducting state to the non-conducting state, the charged voltage on the by-pass capacitor 30 is required to be discharged quickly, therefor, grounding is effected through the resistor 36 and the diode 47 to cause discharging.
The reception band switching operation is performed in the manner below. Firstly, on receiving the low VHF channels, a voltage of, for example, 12 volts is impressed on the terminal 43 with the terminal 42 not impressed, whereby the reverse bias is applied across the switching diode 29 thereby to turn off, so that the coils 26 and 27 are connected in series and, as a result, the tuning circuit 25 has a circuit characteristic needed for reception of the VHF low channels. At the time of receiving the high VHF channels, a voltage of, for example, 12 volts is impressed on the terminal 42 with the terminal 43 not impressed, whereby the switching diode conducts in the forward direction to turn on, so that the coil 27 is short-circuited thereacross by the switching diode 29 and, as a result, the tuning circuit 25 has a circuit characteristic needed for reception of the high VHF channels. Further, on receiving the UHF, no voltage is impressed on the terminals 42, 43 and no power is supplied to the RF amplifier 39, but, only a UHF tuner section 44 is powered to amplify the IF signal of the UHF.
In case the conventional tuner of the foregoing type is equipped in VTR 2 and TV 3 respectively and connected as shown in FIG. 1, there was the drawback that if TV 3 is operated with VTR 2 off, for instance, a jamming signal is generated from VTR 2 in spite of its non-powered state and TV 3 is influenced thereby. Namely, in the non-powered state of the VTR a voltage is not impressed on the tuner 12 within VTR 2. Thus, the switching diode 29 forms a closed circuit through the resistors 34, 35, 37, 38 and an impedance 48 of a power source 50 for the high VHF channels, and no voltage exists across the anode and cathode of the diode 29, i.e., it is zero-biased. The television signal from the antenna 1 is applied through the distributor 5 to the tuner 12 of non-powered VTR 2 and to the tuner 15 of TV 3 being in operation. When the level of the received television signal is extremely high, the television signal flows through the switching diode 29 in the tuner 12 within VTR 2 which is zero-biased as the result of being non-powered. Owing to the rectification action of the switching diode 29 a loop is created so that a rectified current flows through resistor 34, resistor 35, internal impedance 48 of the power source 50, resistor 38 and resistor 37 in this direction, accordingly, the received television signal of a strong level can flow as above. Element 49 represents a current source of power source 50. Because the diode is generally a non-linear element, the diode 29 generates higher harmonics of the television signal when the television signal flows into the diode 29. The generated higher harmonics are applied from the tuner 12 through the distributor 5, high-pass filters 6, 8, 10 within the signal switching device 4 and input into the tuner 15 of TV 3. In such a state, if TV 3 is receiving the UHF television channel, these higher harmonics act as the jamming signal because the frequency band of the higher harmonics extend up to the UHF band. Thus there was the drawback that the normal reception of the UHF television channel is impeded by interference.